Agronomic Crops Team On-Farm Research Projects 1997

Special Circular 160-98

SOYBEAN

Soybean Plow Down Nitrogen

Dr. Jim Beuerlein

A multi-year study was conducted to determine if soybeans planted after
wheat harvest could fix enough nitrogen for a succeeding corn crop to be
an economically viable activity. The positive and negative attributes of
such an activity would be:

Positive

Potentially fix up to 150 lbs. of nitrogen

Reduced nitrogen cost

Reduced need for nitrogen stabilizers

Reduced nitrogen contamination of water resources

Reduced soil compaction due to nitrogen application

Reduced weed seed production following wheat harvest

A potential use for 2.4 million bushels of soybean seed

Recovery of nitrogen left in soil by the wheat crop

Potential reduction in succeeding crop weed control cost

Negative

Cost of soybean seed

Cost of herbicides for weed control

Cost of planting operation

The research project was designed to answer several questions relative
to the production and recovery of a maximum amount of nitrogen:

What relative maturity should the soybean variety have?

Would more vegetative or reproductive growth be more desirable?

Should the residue be incorporated or left on the soil surface?

Would super-nodulating varieties be superior for this purpose?

Procedure

The study was begun with the no-till seeding of six soybean varieties
following wheat harvest in 1993. Maturity Groups 2, 3, 4, and 7 were
represented with one of the group 4 and group 7 varieties having the
super-nodulating gene. Treatments were replicated four times, and plots
were large enough to permit four different tillage systems to
incorporate the residue following the death of plants by freezing.
Tillage systems included no-till, spring disking, fall chisel/spring
disking, and fall moldboard/spring disking.

Within the study, 24 additional plots had herbicides applied but were
not planted to soybeans for the purpose of applying varying rates of
nitrogen the following spring to produce a yield response curve for
nitrogen rate. Corn yields from the soybean residue plots would be
compared to the curve to determine how much corn yield was due to
nitrogen supplied by the soybeans. The study was designed for two years
and was conducted at two locations (Wood County and Union County) each
year.

A soybean seeding rate of 250,000 seeds per acre was applied with a
Great Plains No-Till drill 10-feet wide. Planting was planned to
immediately follow harvest but was typically accomplished about two
weeks after harvest due to rainfall or other reasons. Roundup was used
for a burn down and was applied with appropriate residual weed-control
materials. Hybrid selection and planting rate were determined by the
cooperator.

No nitrogen was applied to the research area with exception of the
non-soybean residue plots which received nitrogen at rates of either 0,
50, 100, 150, 200, or 250 pounds of nitrogen per acre as a 28% nitrogen
solution to develop the nitrogen response curve. Weed control for the
study was accomplished with Roundup and appropriate residual materials.

Of the six attempts to collect meaningful data, only two were
successful. This research program raised more questions than it answered
as is often the case when attempting to create new technology. The
combined results of the two data sets are shown in the accompanying
chart on the next page.

Two important questions were answered (see the chart). First: For
nitrogen to be recovered from this type of system, tillage will be
necessary. Second: Soybean varieties should be of appropriate maturity
to be actively filling pods when they are killed. The rate of nitrogen
fixation is greatest when the plants are producing protein and
depositing it in grain. Both results agree with known biological
principles.

Unfortunately, there were not enough observations to determine the
amount of nitrogen that can be fixed by this system. Another unknown is
what percent of the fixed nitrogen could be recovered the following year
by corn and how much would be left for the second following crop.
Additional research of this nature should be conducted on soil with
lower nitrogen supplying capacities to make the results more definitive
and more easily interpreted. If more research is to be conducted on this
topic, then the number of treatments can be reduced to those where the
soybean variety would be near physiological maturity when killed. The
amount of tillage needed for residue incorporation should be refined.

We wish to thank the Ohio Soybean Council for their support of this
project.